Conventional injection molding machines of the type to which this invention relates include a reciprocating, auger-type plasticating feed screw mounted in a heated barrel for conveying and plasticizing or transforming into a molten state pelletized or granular thermal plastic materials which are fed into the barrel and advanced while being heated to a molten state by the auger type screw to the front end of the barrel whereat a controlled outlet provides fluid communication with a mold. The screw rotates and retracts as the molding material fills the bore space in the barrel between screw end and closed nozzle opening. When a predetermined quantity of molding material is collected in the barrel ahead of the screw, screw rotation is stopped and the screw is forcefully advanced, axially, towards the open outlet. By advancing the feed screw forward towards the nozzle outlet in the barrel, the molding material or shot ahead of the screw is forced or injected from the barrel through the nozzle and into the mold. After injection, screw rotation again starts and the molding material is again collected ahead of the screw in the barrel bore to force the screw to axially retract as pressure builds. The molding sequence is automated and operator variable vis-avis computer commands setting microprocessor controls. Specifically, it is conventionally known practice to control screw rotation and barrel temperature to deposit a predetermined shot in the barrel bore ahead of the screw. It is conventionally known practice to sense shot pressure, screw travel, etc., and control the ram injection pressure (variable or constant) and the rate of flow of molding material into the mold (variable or constant) as well as pressure (ram and mold) control after injection.
In order to insure that the shot is delivered to the mold during the injection stroke, non-return or antibackflow valves mounted to the front of the screw have long been used in the prior art. Such non-return valves are one-way check valves which are typically classified as either a ball check type valve or a sliding ring-type valve. The present invention relates to sliding ring valves.
Ring type valves typically comprise a tip/stud member having a threaded rear end for attachment to the front of the screw and a retainer nose at its opposite end with a rod or stud portion interconnecting the retainer nose with the threaded end. Attached to the rod portion is a valve seat generally adjacent the threaded end. An annular, axially slidable, check ring fits over the rod portion and is sized to fit closely within the barrel. When the valve is assembled, the check ring is free to axially move until one of its ends contacts the retainer end or its opposite end contacts the valve seat affixed to the tip/stud member. When the screw rotates, flow of the molding material advanced by screw rotation axially slides the check ring into contact with the nose-retainer end and material flows past the open valve seat and then between the ring and stud into the barrel bore. During injection, the shot develops pressure against the check ring adjacent the retainer end and moves the check ring to close against the valve seat. Numerous modifications have been made to ring-type valves to enhance or improve their operation.
One such modification, somewhat pertinent to the present invention, may be described as a driven ring valve and is known in the art. For example, Japan Steel Company and Mallard Machine Company offer such valves. In this type of valve, the forward end of the check ring and the rearward end of the retainer nose are in essence serrated so that rotation of the tip/stud member drives or causes the check ring to rotate. The check ring is still free to axially cycle between the retainer nose and valve seat for opening and closing the valve. Because the check ring and tip/stud member rotate together, wear between retainer nose and check ring is virtually eliminated.
In spite of the developments made in microprocessor controls and computer programming now employed to precisely control flow rates, injection speed, shot size, etc., it has been concluded that control variations necessary in precision molding have not been achieved because of inherent variation in shot size. The problem is generally defined in the April, 1987 issue of Plastics Technology at pages 91-95. Secondary valving to improve shut-off operation of the valve is disclosed in the Nov. 1, 1986 issue of English publication Plastics and Rubber Weekly. The same type of a concept is disclosed in assignee's prior U.S. Pat. No. 3,319,299. Simply put, shot size injected into the mold is determined by valve closure and valve closure in ring-type valves is dependent upon unequal pressure build-up acting against the check ring. Slight variations in material, temperature, viscosity, etc., inherently affect valve closure making impossible consistent, repeatable shot size. In certain molding applications, shot sizes less than 1% in variation must be constantly produced. A valve relying solely on pressure differentials to move a fixed distance cannot, inherently, produce consistent closures at the accuracy desired for certain molding applications.
Apart from variation in shot size, it is to be appreciated that, in accordance with conventional practice, some movement of the ram or screw during the injection stroke must occur before pressure at the front of the check ring develops sufficient force to close the conventional check ring valve. During injection forward movement, a portion of the shot material travels past the check ring into the barrel where it remains until the next injection stroke. The injection efficiency of injection molding machines operated in the conventional manner with conventional check rings must always be less than theoretical. The loss of shot material is, of course, dependent upon a number of factors and is not limited only to the density of the material. Tests conducted on an injection molding machine having a maximum shot capacity of 38 ounces demonstrated that when the machine was operated at 19 ounces (one half machine capacity), the loss of shot which occurred during the time the valve closed was about 2.8%. However, many times machines are operated at a shot capacity of only 5% to 10% of maximum capacity. At a shot size indicative of 5% of the machine's shot capacity, the loss of shot to close the valve rises to 28.5% of the shot volume initially accumulated ahead of the check ring valve. The prior art has recognized the throughput loss attributed to valve closing which also results in shot variation. Conventional techniques used with conventional check ring non-return valves have included a pullback technique where, prior to the injection stroke, the screw is axially pulled backwards a slight distance within the barrel. It has been found that use of the pull-back technique makes the check ring more responsive to valve closure upon initiation of the injection stroke with the result that throughput is increased. However, the pull-back technique can cause "splay" on the molded plastic parts causing the parts to be defective. Thus, application of pull-back can be limited and is dependent upon mold design or application.